1. Field of the Invention
The present invention relates to wireless communication systems and to a system and method for implementing asymmetric modulation in such systems.
2. Description of the Related Art
A wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary communication systems include mobile cellular telephone systems, personal communication systems (xe2x80x9cPCSxe2x80x9d), and cordless telephones. The key objective of these wireless communication systems is to provide communication channels on demand between the plurality of subscriber units and their respective base stations in order to connect a subscriber unit user with the fixed network infrastructure (usually a wire-line system). In the wireless systems having multiple access schemes, a time xe2x80x9cframexe2x80x9d is used as the basic information transmission unit. Each frame is sub-divided into a plurality of time slots. Some time slots are allocated for control purposes and some for information transfer. Subscriber units typically communicate with their respective base station using a xe2x80x9cduplexingxe2x80x9d scheme thus allowing for the exchange of information in both directions of the connection.
Transmissions from the base station to the subscriber units are commonly referred to as xe2x80x9cdownlinkxe2x80x9d transmissions. Transmissions from the subscriber units to the base station are commonly referred to as xe2x80x9cuplinkxe2x80x9d transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (xe2x80x9cTDDxe2x80x9d) or frequency division duplexing (xe2x80x9cFDDxe2x80x9d) methods to facilitate the exchange of information between the base station and the subscriber units.
Recently, wideband or xe2x80x9cbroadbandxe2x80x9d wireless communications networks have been proposed for delivery of enhanced broadband services such as voice, data and video. These broadband networks facilitate two-way communication between a base station and a plurality of fixed subscriber units.
Often in such broadband communication systems, multiple schemes are available for modulating and demodulating the transmitted signal. The obvious choice for the modulation scheme is the one that allows the highest transfer bit rate between the base station and subscriber units. However, limitations imposed by the communication system""s design as well as variations in geographic and atmospheric conditions impede use of such high-bandwidth modulation schemes. Furthermore, these variables impact transmissions from the base station to the subscriber unit and from the subscriber unit to the base station differently. Consequently, while it is advantageous for such broadband systems to continuously utilize high-bandwidth modulation schemes, in practice, they often do not.
Thus, there is a need for a system and method which can be implemented in a broadband communication system to optimize the transmission of voice, data and video when multiple modulation schemes are available. Such a system should be flexible so as to account for variations between both the uplink and the downlink in a two-way communication connection between each base station and subscriber unit pair. Moreover, a system that adaptively adjusts the modulation scheme for each individual uplink and downlink according to these variations over time would also be desirable.
One aspect is a wireless communication system for determining a plurality of uplink modulation schemes and a plurality of downlink modulation schemes for use in a wireless communication system including a base station and a plurality of customer premises equipment (CPE), where each of the plurality of uplink and downlink modulation schemes used by each of the plurality of CPE can be asymmetric, such that the uplink modulation scheme may be different than the downlink modulation scheme. The system comprises a plurality of CPE, each including a first modem configured to measure a first link quality based on received downlink data. The system further includes a base station having a second modem configured to measure a second link quality for each of the plurality of CPE based on received uplink data, a first processor configured to receive the first link quality and determine a downlink modulation scheme for each of the plurality of CPE, and a second processor configured to receive the second link quality and determine an uplink modulation scheme for each of the plurality of CPE.
Another aspect is a method for determining a plurality of uplink modulation schemes and a plurality of downlink modulation schemes for use in a wireless communication system which communicates with frames of data and includes a base station and a plurality of customer premises equipment (CPE), wherein each of the plurality of uplink and downlink modulation schemes used by each of the plurality of CPE can be asymmetric, such that the uplink modulation scheme may be different than the downlink modulation scheme. The method comprises determining an uplink quality for a first frame of data transmitted by a CPE and received by a base station, comparing the determined first uplink quality to a plurality of modulation threshold values. If the first uplink quality has crossed one of the plurality of modulation thresholds, a second uplink modulation scheme is selected for the CPE. The method further includes receiving a request for the second uplink modulation scheme at the CPE, determining a downlink quality for a second frame of data transmitted by the base station and subsequently received by the CPE, and comparing the determined first downlink quality to a second plurality of modulation threshold values. If the first downlink quality has crossed one of the plurality of modulation thresholds, a second downlink modulation scheme is selected for the CPE. The method further includes receiving a request for the second downlink modulation scheme at the base station, transmitting a third frame of data by the base station to the CPE using the second downlink modulation scheme, and transmitting a fourth frame of data by the CPE to the base station using the second uplink modulation scheme.